One apparatus of this type as is well known in the art is so constructed that a tightening force adjusting spring is operatively associated with a manually operated dial and the pressing force of the spring is adjustable through rotation of the dial. The thus constructed apparatus has a disadvantage, however, that when it is required to change the tightening force in accordance with the kind of the member to be tightened, it requires interruption of the operation and work in progress because of the manual adjustment of the dial, and takes a long time. This problem is to particularly remarkable where the device is employed with a robot.